In recent years reverse osmosis has attracted a great deal of interest for utilization in fields involving purification of liquids. This is of especial importance when utilizing this sytem in the purification of water and especially saline water. Likewise, the process is also used to remove impurities from liquids such as water or, in the field of dialysis, blood. When utilizing reverse osmosis in the purification of a saline water, a pressure in excess of the osmotic pressure of the saline water feed solution is applied to the solution which is prepared from purified water by the semipermeable membrane. Pure water thereby diffuses through the membrane while the sodium chloride molecules or other impurities which may be present in the water are retained by the membrane. Discussions and descriptions of such treatment in general and the state of art in the middle 1960's is set forth in the treatise Desalinization by Reverse Osmosis, the M.I.T. Press, 1966, which was edited by Ulrich Merten.
Various semipermeable membranes are now being used in commercial processes for performing separations by the reverse osmosis treatment of aqueous solutions either for the portion of relatively pure water or for concentration of a liquid solution being treated or both. Such semipermeable membranes which are being used commercially include the early Loeb-type membranes made of cellulose diacetate by processes such as described in U.S. Pat. Nos. 3,133,132 to Loeb et al. and 3,133,137 to Loeb et al. The Loeb-type membranes comprise the asymmetric type which are characterized by a very thin, dense surface layer or skin that is supported upon an integrally attached, much thicker supporting layer. Other types of semipermeable membranes which are also in use include membranes having been fabricated from polyamides, polyimides, polyphenyl esters, polysulfonamides, polybenzimidazole, polyarylene oxides, polyvinylmethyl ether and other polymeric organic materials.
It is taught in U.S. Pat. No. 4,243,701 to Riley et al. that certain membranes may also be utilized for the separation of various gases. The separation of a gas mixture utilizing a membrane is effected by passing a feed stream of the gas across the surface of the membrane. Inasmuch as the feed stream is at an elevated pressure relative to the effluent stream, a more permeable component of the mixture will pass through the membrane at a more rapid rate than will a less permeable component. Therefore, the permeate stream which passes through the membrane is enriched in the more permeable component while, conversely, the residue stream is enriched in the less permeable component of the feed.
The use of adsorbents or molecular sieves in separating components from fluid mixtures is also long known. In the adsorption type separation process the adsorbent exhibits selectivity of one mixture component over another or, with a molecular sieve, one component is more retained than another. The adsorbent may be employed in the form of a dense compact fixed bed which is alternatively contacted with the feed mixture and desorbent materials. In the simplest case, the adsorbent is employed in the form of a single static bed in which case the process is only semicontinuous. In another embodiment, a set of two or more static beds may be employed in fixed bed contacting with appropriate valving so that the feed mixture is passed through one or more adsorbent beds, while the desorbent materials can be passed through one or more of the other beds in the set. The flow of feed mixture and desorbent materials may be either up or down through the adsorbent.
The most commercially successful embodiment of the adsorptive type separation process is the countercurrent moving-bed or simulated moving-bed countercurrent flow system. In that system the adsorption and desorption operations are continuously taking place which allows both continuous production of an extract and raffinate stream and the continual use of feed and desorbent streams. The operating principles and sequence of such a flow system are described in U.S. Pat. No. 2,985,589 to Broughton et al.
There are numerous references which disclose the incorporation of various materials with separation membranes. U.S. Pat. Nos. 3,457,170 to Havens; 3,878,104 to Guerrero; 3,993,566 to Goldberg et al.; 4,032,454 to Hoover et al.; and 4,341,605 to Solenberger et al. teach the use of structural supports or reinforcement fibers or fabrics to aid the membrane in resisting the high pressures used in the reverse osmosis process. U.S. Pat. No. 3,556,305 to Shorr shows a "sandwich" type reverse osmosis membrane comprising a porous substrate covered by a barrier layer, in turn covered by a polymer of film bonded to the barrier layer by an adhesive polymeric layer. U.S. Pat. No. 3,862,030 to Goldberg shows a polymeric matrix having an inorganic filler such as silica dispersed throughout which imparts a network of microvoids or pores of about 0.01 to about 100 microns, capable of filtering microscopic or ultrafine particles of submicron size. U.S. Pat. No. 4,302,334 to Jakabhazy et al. discloses a membrane "alloy" comprising a hydrophobic fluorocarbon polymer blended with polyvinyl alcohol polymer which imparts hydrophilic properties to the membrane. U.S. Pat. No. 4,230,463 to Henis et al. discloses multicomponent membranes useful for separating gases comprising a polymer coating on a porous separation membrane which also may be a polymer such as a polysulfone. This patent indicates that a ratio of total surface area to total pore cross-sectional area must be at least 1000:1 as specifically set forth in claims 5 and 19. This means that the porous polysulfone which is commonly used as an ultrafiltration or reverse osmosis support cannot be utilized inasmuch as the commonly used porous polysulfone has a total area/pore cross-sectional area ratio in the range of from about 5:1 to about 900:1. In this respect, it is to be noted that the higher the ratio, the smaller is the pore diameter, i.e., a tighter membrane. As will hereinafter be shown in greater detail in the examples at the end of the specification, the relatively greater porous supports, that is, those which possess a low ratio are desirable for coating the polysulfone with a combination of silicone rubber and a glycol plasticizer. This is in contrast to prior references in which a highly porous polysulfone is coated with only silicone rubber, thus leading to a low selectivity. An additional difference which exists between the membrane of U.S. Pat. No. 4,230,463 and the membrane of the present invention is the unexpected stability of the latter with regard to selectivity. The selectivity which is enjoyed at the outset of the separation process utilizing the membrane of Henis et al. will be effective for only a relatively short period of time, inasmuch as the pressure difference will rapidly cause a deterioration of the membrane with an attendant loss of selectivity and stability.
U.S. Pat. Nos. 2,673,825, 3,532,527 and 4,454,085 disclose membranes which may be utilized in separation processes and involve the use of a glycol in the preparation of the membrane. However, in these three patents, the glycol is coagulated with water in the casting solution and therefore most, if not all, of the glycol is leached out before separation. Therefore, the glycol does not become an integral part of the membrane as is present in the membrane of this invention.
Mixed matrix membranes such as molecular sieves incorporated with polymeric membranes are also broadly disclosed in the art. In the article "The Diffusion Time Lag in Polymer Membranes Containing Adsorptive Fillers" by D. R. Paul and D. R. Kemp, J. Polymer Sci.; Symposium No. 41, 79-93 (1973), the specific mixed membrane used was a Type 5A (Linde) zeolite incorporated with a silicone rubber matrix. The Paul et al. article illustrates that the zeolite "filler" causes a time lag in reaching steady state permeation of the membrane by various gases due to the adsorption of the gases by the zeolite. It is taught in this article that once the zeolite becomes saturated by the permeating gas, a steady state rate of permeation through the membrane is reached so that the membrane selectivity is essentially the same as if the zeolite was not present. The Paul et al. article teaches making the mixed membrane by dispersing the molecular sieves into the fluid silicone prepolymer prior to casting.
We have discovered novel multicomponent membranes, their method of manufacture and uses for which they are uniquely suitable not disclosed by any of the known art either alone or in combination.